Selection and use of laboratory tests in the rheumatic diseases

Current clinical practice relies heavily on serologic testing for the prompt and accurate diagnosis of rheumatic diseases. Serologic testing should be used to support the findings of the history and physical examination, and, in some cases, to monitor disease activity. The inflammation of the rheumatoid arthritis (RA), polymyalgia rheumatica, and temporal arteritis can be assessed by the erythrocyte sedimentation rate (ESR). The C-reactive protein (CRP, an acute-phase protein) test, which is newer, correlates more closely than ESR with clinical and radiographic parameters of RA inflammation. The rheumatoid factor test is nonspecific as a screen for RA, and some argue that it is also insensitive (accounting for the existence of "seronegative" RA). High titers of rheumatoid factor are associated with progressive joint inflammation, erosions, and disability. Antinuclear antibody (ANA) tests are likewise nonspecific, but ANA subtypes have proved to be very specific for subtypes of connective tissue diseases. Examples are the presence of anti-DNA antibody in systemic lupus erythematosus; anti-centromere antibody in the CREST syndrome of scleroderma; anti-histone antibody in drug-induced lupus; and anti-Ro antibody in neonatal lupus. Anti-neutrophil cytoplasmic antibodies (ANCA) are a new group of auto-antibodies seen in Wegener's granulomatosis. Brief case descriptions are presented to illustrate appropriate selection of these antibody tests as well as tests for antiphospholipid antibodies and cryoglobulins.